1. Field of the Invention
The present invention relates to a calibration method, and more particularly to an effective calibration method used by a transparency scanner.
2. Description of the Related Art
Currently, a plate scanner, to scan a transparency, may use a scanning mask. However, the transparency, including a positive film or a negative film, has a wider exposure margin than a non-transparent object due to the film substrate, and requires longer exposure duration. Thus, when scanning the transparency, the plate scanner must be calibrated with increased exposure duration. Typically, there are two methods for calibrating a scanner. The first calibration method results in excessive exposure duration, easily causing a saturation of the scan signal. The second calibration method places transparent tape above a calibration area. This method, however, has a complex design, higher manufacturing cost, and low precision.
FIG. 1a is a block diagram of a conventional scanner. FIG. 1b is a schematic figure of an object placed on the platen of the scanner of FIG. 1a. In FIG. 1a and FIG. 1b, a glass platen 120 is installed between a lamp 102 and a charge-coupled capture device 104, and has a calibration area 122 and a scanning area 124, and a transparency 126 is placed on the scanning area 124.
When scanning the transparency 126, the calibration area 122 is also scanned according to the same exposure duration, to produce an analog scan signal SA and an analog calibration signal, respectively. A signal processing device 106 then converts the analog scan signal SA into a digital scan signal SD. The image processing device 110, then receives the digital scan signal DA, for related image processing.
To prevent data loss, however, the analog scan signal SA must be amplified by a gain coefficient before conversion, wherein the gain coefficient is calculated by the calibration signal.
FIG. 2 is a curve of a scan signal of the scanner of FIG. 1a. In FIG. 2, a scan signal 22 is a signal output from the charge-coupled capture device 104, and a signal 24 is a product of the scan signal 22 and a gain coefficient. The amplitude of the scan signal 22 is between 0 to M1, and a range of a digital scan signal converted from the scan signal 22 is between 0 to D1. Commonly, an ideal range of the digital scan signal is between 0 to D2, while D2 is the maximum digital value after the conversion. The maximum digital value, for example, of an 8 bit signal processing device is 255 or near 255 calculated by 28−1.
Thus, to obtain ideal digital signal range, the original scan signal 22, must be amplified into the signal 24 having an amplitude range between 0 to M2 by the calibration signal before conversion.
The exposure duration of scan signal 22, however, is affected by different film substrates. Hence, when using the same exposure duration to scan the calibration area and the transparency 126, according to the related art, a suitable gain coefficient for amplifying the scan signal 22 to be converted into the digital scan signal having the maximum digital value D2 cannot be obtained.
Therefore, the present invention provides a calibration method using two different exposure durations to obtain a suitable calibration signal and a scan signal, respectively. The scan signal is then calibrated according to the calibration signal thereby obtaining a digital scan signal having a maximum digital value.